There are a variety of optical signal sources used for creating carrier signals for optical signal-based telecommunication systems. One type of signal source is a semiconductor laser that has certain advantages in being easy to manufacture in large quantities at a reasonable cost. There are many different types of such semiconductor laser signal sources including edge emitting semiconductor lasers, vertical cavity surface emitting lasers and more recently horizontal cavity surface emitting lasers.
Low-cost signal sources are urgently required to extend the optical-based networks from the present long haul backbone ring portions to local nodes closer to the end user, the latter being the so-called ‘metro’ portion of the network. While large and expensive signal sources are justifiable in the long haul backbone portions, the same cannot be said for all of the internal network nodes in a metro area. Infilling the network, bringing the optical signals closer and closer to the end user, is conditional upon being able to provide low cost high quality signal sources in large volumes to provide the vast number of required carrier signal sources for the metro portion of the network. To date there have been efforts to provide such signal sources, but the prior art signal sources suffer from numerous disadvantages and thus have not been deployed in a widespread fashion.
No matter what form the optical signal emitter takes there is a need to couple the signal light output to an optical waveguide, such as an optical fibre. Much effort has been devoted to designing the signal source or emitter to yield an output signal of sufficient power and signal quality that it can be efficiently and effectively coupled to the fibre or the like. Thus, the art teaches various designs of semiconductor which are claimed to produce a Gaussian shaped far field signal which can be easily and efficiently coupled to a fibre.
An electro-optical interface can be considered to be a point in a telecommunications network where the electrical signals are converted into optical signals in one direction and optical signals are converted to electrical signals in the other direction. A major concern in developing an appropriate signal source for the metro network portion of an optical network is the cost of the signal source. This is because there is a need for a separate signal source for each optical channel of the telecommunications band and separate signal sources at each electro-optical interface in the network. A low cost design is therefore required if the general deployment of such optical signal sources is to occur. The cost of the signal source can be divided into two main components. The first is the actual capital cost of the optical signal source. The second is the packaging cost of packaging the signal source together with whatever other components may be required to couple the signal source to a waveguide such as a fibre or the like.
Optical feedback is a known phenomenon, which can have a dramatic effect on the performance of laser signal sources. This property can be used to improve the optical signal output, through a so-called external cavity configuration. For example, if the longitudinal mode selection of the laser is made by other means, such as a grating reflector, the external feedback may be used for tuning the laser emission frequency or for a considerable line width narrowing. An external cavity having controlled external reflection or feedback may also be useful for reducing the signal chirp. Thus, prior art designs have made use of controlled feedback to improve signal characteristics in some cases. Usually such feedback, while leading to lower chirp and narrower lines, will also result in a lower bandwidth making the laser more difficult to modulate at high rates.
While controlled optical feedback may be useful as noted above, uncontrolled optical feedback can have the opposite effect. Uncontrolled optical feedback, also referred to as back reflection, can arise from any interface or scattering center in an optical network. Common sources of detrimental feedback include simple interfaces or partially reflective surfaces associated with a fibre pigtail connection in an optical network. Back reflection can travel along an optical path or waveguide and need not originate in the immediate environs of the signal source. When back reflected signal light couples back into the laser cavity, changes in the resonance condition arise causing often significant changes to the laser output. A back reflection into a laser cavity can also be considered as setting up a second cavity having many more modes than the original. Detrimental effects to signal quality include instabilities in the output signal power, mode hopping, wavelength shifts, increased noise and increased spectral line width of up to several tens of Gigahertz. These instabilities are also referred to as “coherence collapse” and arise when the back reflection enters into the laser cavity and detrimentally affects the lasing phenomenon.
To avoid the uncontrolled effects of back reflection, an optical isolator is required in all but the least demanding of applications. An optical isolator is defined as an element that allows light to pass in one direction only and is typically deployed to allow the outgoing signal light to pass while preventing back reflections from passing and entering into the cavity thus disturbing the laser stability. Isolators are normally placed between the signal source and the optical fiber or waveguide. Typically a lens is placed prior to the isolator to collimate the light output from the optical signal source through the isolator and a second lens is used to couple the light into the optical fiber or waveguide. The isolator must be placed and configured in a way to prevent detrimental back reflections from entering into the laser cavity, since such back reflections can cause the unwanted changes to the cavity characteristics and the loss of signal quality noted above.
The need for an optical isolator as explained has several undesirable consequences. Firstly, the further the signal source is away from the end of the fibre with which it is to be optically coupled, the more difficult and precise the alignments of the intervening components need to be. The more difficult the alignment, the more difficult the packaging becomes, thus lowering the manufacturing yields and increasing the packaging expense. The complexity of alignment increases nonlinearly with the number of elements required as the alignment errors in each element are cumulative for the overall alignment. Lastly of course there is the actual cost of the additional components, such as the isolator and any required lensing that is in many cases even more costly than the signal source.
FIG. 1 shows a typical Coarse Wavelength Division Multiplexed (CWDM) optical signal source 10, in the form of a laser semiconductor chip coupled to a fiber waveguide 12 according to the prior art. The laser chip 10 is of the edge emitting type and is mounted behind a lens 14, followed by an isolator 16. A back facet detector 18 is also shown, for monitoring the power of the output signal (since the signal is emitted from both edges of the chip). It will be seen that the fibre 12 is mounted to receive the focussed and isolated output or carrier signal from the chip 10. It will also be noted that stands 20 and 22 need to be positioned relative to chip 10 to couple the signal to the fibre 12. Electrical connectors 24 are also shown.
FIG. 2 shows a typical Dense Wavelength Division Multiplexed (DWDM) edge emitting signal source 30 coupled to a fibre waveguide 32 also according to the prior art. The general configuration is similar to that shown in FIG. 1, with additional components to ensure very stable signal output wavelengths. Thus, a thermoelectric cooler (TEC) 34 is provided for accurately controlling the temperature of the signal source. A back facet detector 36 is provided as well as a lens and isolator assembly 38. In this case two ball lenses 42, 44 are used to couple the output signal into the fibre 46. As can be appreciated the arrangement of these components which are already very small and which are likely to become even smaller in the future, represents a significant packaging challenge and expense.
What is needed is an optical semiconductor signal source packaging configuration which on the one hand reduces alignment concerns and hence packaging costs, and yet on the other hand produces a stable output suitable for use as a signal carrier wave in optical networks.